RF transistors are designed to handle high-power radio frequency (RF) signals in devices such as stereo amplifiers, radio transmitters, television monitors and other devices. RF transistors consist of an N-type or P-type layer sandwiched between two layers of the opposite type. Like other semiconductor devices, RF transistors are made of semiconductor materials such as silicon (Si), silicon-germanium (SiGe), Galium Arsenide (GaAs), etc. and doped with impurities to induce changes in electrical properties. The junctions between the semiconductor sections cause a weak input to be amplified. Varying the current between a gate and a source of the transistor varies the current flow between the source and the drain.
A power metal oxide semiconductor (MOS) or bipolar transistor switch or amplifier, fabricated on a wafer, has a large periphery or surface area. The large periphery is required to maintain a high device current and a low ‘on’ state device impedance. Such large periphery devices can have significant common area with the substrate, since the device source and drain contacts must have reasonable width for low resistance contacting. Because of this large periphery, RF transistors suffer from losses, due to parasitic capacitances when integrated on silicon foundry substrates, particularly on highly doped substrate regions. The extensive area to the substrate causes high coupling to the ground plane, which contains lossy silicon doped material. The capacitive losses can lead to degradation in RF performance, such as RF power max-frequency (Fmax) performance.